1. Field of the Invention
This invention relates to apparatus for measuring fluid pressures including differential fluid pressures. More particularly, this invention relates to improvements in means for calibrating pressure measuring apparatus to assure accurate readings, and also to improvements in means for diagnosing the condition of such apparatus.
2. Description of the Prior Art
Instrumentation systems for use with industrial processes have employed apparatus of various kinds for measuring fluid pressures, especially differential pressures such as are produced across an orifice plate in a flow pipe for the purpose of developing a fluid flow-rate signal. U.S. Pat. No. 4,165,651 to Everett Olsen shows a differential pressure transmitter of advanced design, employing a vibratable wire tensioned in accordance with the differential pressure being measured so that the frequency of vibration provides an accurate measure of the differential pressure. Still other devices are available commercially based on different principles, such as the use of strain-gauge IC chips for sensing applied pressures.
Pressure measuring instruments often are installed in places where they are subject to widely varying environmental conditions, such as changing ambient temperatures. Consequently, it is not uncommon for the instrument zero-set and span calibration to drift or in some way be offset, resulting in erroneous readings. Since the instruments frequently are in locations which are not readily accessible for routine maintenance, zero-set and calibration errors in many cases have not been easily correctible by operating personnel. Moreover, calibrating the span of instruments of the kinds available heretofore typically has involved relatively complex and time-consuming procedures.
Because of the importance of minimizing measurement errors, various proposals have been made for solving or ameliorating these problems. For example, remotely-operated zero-set apparatus now is available for use with differential-pressure transmitters. Such apparatus comprises a remotely-controllable pressure manifold which, upon command, blocks the low-pressure process line and by-passes the high-pressure process line to the high and low sides of the transmitter, producing a zero differential-pressure condition. If under such circumstances the transmitter output signal differs from that indicating zero differential pressure, the error is stored in memory and thereafter is used (as by means of a microprocessor) to correct the output signal when measurements are resumed.
However, such remote-set of instrument zero does not correct for errors in span calibration. Thus, in an effort to avoid the effects of such errors, differential pressure-transmitters have been designed to include one or more condition-sensing elements (such as temperature and static pressure sensors) arranged to function with associated devices to automatically adjust the transmitter output signal in response to changes in the sensed conditions. For example, the transmitter output signal may be controllably altered in accordance with a predictive algorithm stored in a microprocessor forming part of the instrument.
Although such compensator arrangements improve the accuracy of the pressure measurement, they have not satisfactorily solved the problem. In part, this is because such techniques are not capable of achieving the desired accuracy, particularly since there remain other uncompensated variables. Thus, the need for instrument re-calibration from time to time is not eliminated. Moreover, such compensator arrangements are relatively costly to implement.
In copending application Ser. No. 06/661,807, filed by George E. Sgourakes on Oct. 17, 1984 (and assigned to the assignee of this application), there is shown a pressure measuring instrument having means for monitoring the span calibration so that such calibration can be reset as necessary. This span calibration means comprises a reference pressure device for developing and applying to the instrument a reference pressure pulse of precisely-controlled magnitude. In the preferred embodiment, the device includes a vertically-oriented cylindrical tube containing a fill-liquid which communicates with fill-liquid in the instrument. A solid metal ball in the tube can be raised to the top of the tube (as by an externally-developed magnetic field) and then released. The ball drops under the force of gravity through the fill-liquid to produce a pressure pulse of essentially constant magnitude over much of the drop distance. This pressure pulse is used as a reference pressure for setting the span calibration of the instrument, as by adjusting electronic circuitry forming part of the instrument, or located remotely from the instrument.
One important advantage of the invention disclosed in said copending application is that it makes possible remote calibration of the instrument. Such calibration moreover is quite accurate, and can be made while the instrument is under static process pressure.
It has been found advantageous for certain calibration needs to provide a reference pressure pulse of magnitude substantially larger than can be produced by the device shown in said copending application. It is the purpose of this invention to provide means for developing such larger magnitude pressure signals, and to effect other improvements as will be described.